The invention relates generally to modems in which serial data modulates a carrier which in turn is transmitted over lines, such as telephone lines, to a receiver where the data are reproduced. In particular, the invention relates to carrier current data transmission systems in which modems utilize AC power lines for the transmission of the carrier. Carrier current receivers in association with carrier current transmitters often comprise a complex system consisting of numerous uniquely addressable remote receivers or transceivers in communication with and under the control of a master unit. Such systems commonly employ FM transmission to alleviate problems associated with power line noise.
In any such system in which a digitally modulated carrier must be demodulated, a comparator is employed to "slice" the demodulated baseband data into clean digital ones and zeroes. The success of the comparator is crucial to the optimum performance of the system and is in turn dependent upon the level of its threshold relative to the level of the demodulated signal. If the threshold is set midway between the peak-to-peak excursions of the signal, the comparator will perform best. Any deviation from the above condition is considered less than optimum and will result in reduced noise immunity and/or data pulse width distortion. Noise and/or harmonic reduction filters are often placed ahead of the comparator and they may contribute to the potential pulse width distortion due to their "rounding" effect on the data pulses.
Many schemes have been used to center the comparator threshold relative to the incoming signal, which is often itself contaminated with unpredictable DC offsets. An example would exist in the case of a PLL receiver in which the difference between the center frequency of the transmitter and the center frequency of the receiver VCO B08200210/km/rg manifests as DC offset at the receiver demodulator output. One way is to directly couple the signal to one of the comparator inputs and couple the capacitively derived average of the signal to the other input. The comparator threshold is thus the average of the signal irrespective of the signal amplitude. Unfortunately, if the data stream has a duty cycle other than 50%, then the average value will not be midway between the peak-to-peak excursions of the signal. Also, a relatively large capacitor must be used to effectively average the long strings of ones and zeroes. Such a capacitor cannot be quickly charged and discharged as is necessary for frequent transmit/receive exchanges. An alternative approach involves using a DC feedback loop to force the comparator threshold midway relative to the signal. But once again, the error storage capacitor for the loop will respond to the average of the signal resulting in an erroneous threshold value for non-50% data streams. Some systems have used peak detection schemes that attempt to accurately peak detect a small amplitude and noisy demodulated signal which often sits on top of a large and varying common mode DC voltage. Obvious problems result from the peak detector's tendency to detect noise and its inability to accurately hold the peak value. Due to the relatively large common mode voltage, this value is often several volts-meaning that the holding capacitor droop is a large percentage of the much smaller signal value. Large capacitors are needed to reduce droop and adversely affect transmit/receive switchover time.